Method of recovering zirconium and hafnium



United States Patent 3351, 325 METHOD OF REQOVERTNG ZIRQONHUM ANDHAFNIUM William H. Owens, Tullahoma, Tenn., assignor, by mesneassignments, to Pittsburgh, Plate Glass Company, Pittsburgh, Pa, acorporation of Pennsylvania No Drawing. tContinuation of applicationSer. No. 93,872, Mar. 7, 1961. This application Mar. 17, 1966, Scr. No.535,031;

12 Claims. (Cl. 23-23) ABSTRACT OF THE DISCLOSURE Alkali metal salts ofzirconium or hafnium containing silica are subjected to the treatment ofa concentrated aqueous acid solution while the silica therein is in anundissolved, solid and substantially unhydrated state to dissolve thesalt in the acid solution which is then recovered from the solid silica.

This application is a continuation of application Ser. No. 93,872, filedMar. 7, 1961 and now abandoned.

This invention relates to a novel method of recovering zirconium andhafnium values from compounds or compositions containing thesematerials.

Silica and other impurities conventionally occur in nature withzirconium and hafnium. One of the more widely known zirconium ores iszircon, which has the composition ZrO .SiO and which contains a smallamount of hafnium, probably as HfO .SiO Thus, this material containsapproximately one mole of SiO per mole of ZrO and HfO The ore may alsobe contaminated with additional silica which is present as ordinarysilica sand and may contain small amounts of other impurities, such asoxide of titanium, iron, columbium, thorium, uranium or other heavymetals.

According to one of the processes which has been proposed for recoveryof zirconium and hafnium from such ores, the ore is heated with analkali metal hydroxide or an alkali metal carbonate at an elevatedtemperature sufficiently high to convert the silica to a water solublephase and to produce an alkali metal zirconate. Thereafter, theresulting product is extracted with Water to remove part of the silica,apparently in the form of a water soluble alkali metal silicate. Theremaining solids, which contain a high concentration of water,essentially all of the zirconium, the residue of the silica (rarely inexcess of about 30 percent thereof) and essentially all of the hafniumare treated with acid, such as sulfuric acid or like acid, to dissolvethe zirconium and hafnium.

The silica is dispersed through the resulting solution in a very highlycolloidal state. In this form, it is very slow to settle and has adefinite tendency to gel, particularly when the solution is contactedwith an organic solvent. Since Zirconium frequently is separated fromhafnium by solvent extraction, this is a very objectionable phenomenon.

This silica appears to be in the form of very fine silica particles,probably below 15 millimicrons in size, which are very highly hydrated.Silica of this character is either soluble in the aqueous solution orforms a relatively stable colloidal suspension. It tends to dry in theform of hard, flinty particles which resemble glass in character. Inhigh enough concentrations, it can cause the entire solution to convertinto a gel or jelly. This is particularly observed when the solution issubjected to extraction with organic solvents.

In order to deal with this problem, it has been proposed to digest theaqueous solution thus obtained with acid in certain high concentrationsin order to precipitate the silica and to convert it to a morefiltcrable form. While this can be done, the zirconium, during thistreatment, has a tendency to convert to an insoluble form; consequently,some portion of the zirconium often becomes insoluble.

According to the present invention, many of the difii culties heretoforeencountered may be minimized or even substantially eliminated. In theprocess herein contemplated, the alkali metal zirconate containingsilica is subjected to the treatment of an aqueous acid solution whilethe silica in the zirconate in contact with or in association with thealkali metal zirconate is in undissolved or solid state. Preferably,this silica is substantially unhydrated.

Thus, it has been found, according to this invention, that if the silicain the alkali metal zirconate is in solid state and particularly if itis essentially unhydrated and undissolved, the alkali metal zirconatecontaining this silica can be contacted with acid under conditions suchthat the reaction mixture is acidic substantially throughout the entireperiod of reaction (or at least in the initial stages until the silicabecomes permanently insoluble) and the silica is then converted to aninsoluble, readily filterable form.

This silica is pulverulent in character as distinguished fromgelatinous. The major portion thereof has a particle size of above about15 millimicrons, much of it being in the form of flakes, flocs orrelatively porous granules. As a consequence, the silica will rapidlyseparate from solution, can be readily filtered out of solution, anddoes not seriously tend to cause the solution to gel.

In the practice of the process herein contemplated, it is foundimportant to maintain the concentration of acid high" enough to hold thesilica out of solution. Usually the acid concentration should be highenough to maintain the pH of the mixture below about one throughoutsubstantially the entire period of reaction. Thus, for most purposes,the alkali metal zirconate is added to a pool of the acid, care beingtaken to insure that the pool is maintained acidic, the concentration ofthe pool generally being held above about 3 moles of acid per liter ofsolution.

Various acids (usually inorganic or mineral acids) which form watersoluble zirconium salts preferably having a solubility in water or acidsolution of at least 25 grams per liter may be used for this purpose.Typical acids which are suitable are: nitric acid, sulfuric acid,hydrochloric acid, hypochloric acid, perchloric acid, and the like.Organic acids, such as trichloroacetic acid, may be used but are moreexpensive. Especially useful is nitric acid because it produces asolution of zirconium nitrate and hafnium nitrate which can readily beextracted directly and without further processing for separation of thezirconium from the hafnium.

The initial water content of the reacting mixture is maintained low,usually below 5 percent and for most purposes not over about 0.5 to 1percent by weight based upon the weight of the initial solids subjectedto treatment. Thus, the frit produced by reacting zircon with alkali isreacted with the acid before it absorbs moisture in excess of 5 percent,preferably before it absorbs 0.5 to 1 percent by weight of moisture. Inthe acid reaction, the initial water content of both the frit and theacid is low. Of course, some water in addition to this initial watercontent is generated in the course of the reaction. Evolved water can bedistilled off as formed if desired. Thus, the total water in thereaction mixture rarely exceeds 40 percent by weight and normally is solow that a large portion of alkali metal nitrate or other alkali metalsalt formed by reaction of the acid with the alkali of the fritprecipitates or crystallizes out of solution.

The process may be conducted in several ways. For example, the alkalimetal zirconate frit which is produced by reaction of alkali metalhydroxide with zircon may be reacted directly with the acid withoutprevious contact with water, i.e., in an essentially anhydrous state, todissolve sodium silicate. Thus, the frit produced by reaction of alkalimetal hydroxide with zircon substantially as described in US. LettersPatent No. 2,962,346, may be extracted directly with an inorganic acid,such as nitric acid, by feeding this frit either hot or cold to a poolof the nitric acid or like acid.

Usually, the temperature of the pool of acid should be high, above roomtemperature. For most purposes, the temperature of the pool shouldexceed 50 C. and advantageously the nitric or like acid may be at itsboiling point. The rate of addition of the frit to the acid may beconveniently controlled to maintain this temperature by the heat ofneutralization evolved upon addition of the frit to the nitric acid.

In the practice of this process, the frit is usually added to an acid ofhigh concentration. For example, frit may be added to a boiling solutionof nitric acid containing 40 to 60 percent by weight or more of HNO Inthe course of addition, a portion of the nitric acid becomesneutralized. Usually, however, addition of frit is discontinued beforethe nitric acid concentration falls to about 30 percent by weight of theresulting aqueous solution.

The frit may be added to the nitric or like acid solution while it ishot and substantially at the temperature at which it is released fromthe reactor in which it is formed. Normally, the reaction of sodiumhydroxide or like alkali metal hydroxide with zircon takes place at atemperature well above 300 0., usually in the range of 4SO650 C. Thus,the frit produced in this reaction is usually at these temperatures oronly slightly below. Thus frit may, without cooling, be introduced intothe nitric acid. Alternatively, the frit may be cooled at any convenienttemperature, for example, room temperature or above.

If the frit is allowed to cool, however, it will inevitably pick up acertain amount of moisture from the atmosphere. The amount of moisturewhich it will pick up will vary to a considerable degree, depending uponthe atmospheric humidity and also upon the length of time it is allowedto stand. For most purposes, caution should be taken to avoid absorptionof the water to cause a portion of the alkali metal silica zirconate inthe frit to dissolve in water. This can be avoided if the temperature ofthe frit added to the acid is maintained above about 75 to 100 C.

As a consequence of the process herein contemplated, there is producedan aqueous solution of the zirconate salt of the acid, for example,zirconyl nitrate, in which there is dispersed precipitated silica andusually the precipitated alkali metal salt of the acid, such as sodiumnitrate. This is particularly true for the acid solution is maintainedstrong enough to insure production of a solution having such a limitedwater content as to avoid dissolution of all of the evolved sodium salt,such as sodium nitrate.

The silica which is present in this solution can be filtered withextreme ease. A clear supernatant liquor containing zirconium nitrate orlike zirconium salt can be decanted from a major portion of the silicaand other suspended solids with relative ease.

Thus, the solution can be separated from the suspended solid bydecantation and/ or filtration. Residual liquor in contact with theresulting filter cake or thickened slurry of solids can be washed fromthe solid if desired or if necessary. However, this results in adilution of the zirconium salt since the wash liquors are usually muchmore dilute than the initial solution. Recovery of the zirconium saltsand/or the alkali metal salts from such washed liquors can be effected,but such recovery often is very expensive.

According to a further embodiment of this invention, it has been foundthat substantially complete removal of zirconium and/ or hafnium in anaqueous solution containing suspended solids of this character (silicaand/ or alkali metal salts, such as sodium nitrate) can be achieved byextracting the solution and/ or the slurry of suspended solids directlywith an organic solvent, such as tributyl phosphate or like solventwhich extracts zirconium. By this means, it has been discovered that thezirconium and/or halfnium may be substantially completely removed in theorganic solvent leaving impurities, including silica as well as othermetallic impurities, such as iron, tantalum, niobium, chromium, and thelike, either dissolved or suspended in the aqueous solution. It ishighly surprising that this process is feasible.

Thus, when an aqueous solution of zirconium salts, such as Zirconiumnitrate, containing dissolved silica or silica suspended in colloidal orhydrated form (such as has been produced by processes heretofore used)is extracted with tributyl phosphate, this solution normally converts toa gel, such as a solid or semi-solid. Little or no separation of organicsolvent can be achieved. Even when the silica concentration is low, thesilica component tends to concentrate at the interface between theorganic solvent and the aqueous layer and to impair separation of theorganic solvent from the aqueous phase. For this reason, it has beenconsidered necessary to conduct extractions of this character in thesubstantial absence of silica. Rarely has it been considered feasible toconduct extraction of solutions in which the Si content exceeded about0.1 percent by weight based upon the zirconium content of the solution.

In contrast, applicant has discovered surprisingly that if the silica isin a pulverulent form of the type produced and herein contemplated asdistinguished from the gelatinous or colloidal form, the silica does notcause gelation of the resulting mixture upon extraction with organicsolvents, such as tributyl phosphate, but on the contrary remainsassociated with the aqueous phase. Consequently, all or substantiallyall of the zirconium and hafnium content can be extracted into theorganic phase with very little difficulty and without the dilution whichis attendant to washing of filter cakes to recover dissolved zirconiumentrapped therein.

This extraction can be conducted in a simple manner simply by mixing thesolvent with the aqueous solution produced by reaction of the frit withthe nitric acid as described above and separating the organic phase. Ifdesired, this can be repeated one or two times in order to ensurecomplete removal of the zirconium from the solution. However, a singleextraction usually is sufiicient.

The amount of solvent can be readily controlled so as to causeextraction of substantially all of the zirconium and hafnium withoutextraction of the other metal impurities. The amount of solvent used, ofcourse, is dependent to a large degree upon the concentration ofzirconium and hafnium in the solution as well as the concentration ofthe impurities. It depends also upon the nature of the solvent. In likemanner, the number of stages of extraction which is used determines to alarge degree of the amount of solvent required. Thus, more solvent isrequired for a single stage of extraction which is resorted to thanwould be required where the extraction is conducted in a column where aplurality, say five or more stages of extraction, takes place. Generallyabout one volume of solvent per volume of aqueous medium is enough.However, the exact amount required can be determined readily by simpleexperimentation in which the weight ratio of impurities (metals otherthan hafnium and/ or zirconium) to zirconium and hafnium in thenonaqueous phase is measured. If this ratio is not lower, preferablysubstantially lower, than the relative ratio of such impurities tozirconium and hafnium in the aqueous solution, the volume of solvent pervolume of aqueous solution should be reduced until such a reduction inimpurity in the concentration can be achieved.

The separation of the organic solvent (or organic phase) from theaqueous phase is especially effective when conducted in the presence ofsolid suspended alkali metal salt since, even when small amounts ofgelatinous silica are present, the solid salt minimizes the tendency forthe silica to collect at the interphase or to gel and, thus, separationof the organic from the aqueous phase is facilitated. Sodium nitrate orlike salt which is crystallized from solution during the acid treatmentof the frit is especially useful for this purpose.

Various solvents which form two liquid phases with water, i.e., areinsoluble with water in some proportion and which extract the zirconiumsalt such as zirconyl nitrate can be used for this purpose, includingthose mentioned in US. Patent No. 2,753,250 granted to H. A. Wilhelm etaL, July 3, 1956. Included are the liquid trialkyl phosphates, such astributyl phosphate, tricresyl phosphate, trihexyl phosphate, trioctylphosphate, oxidecyl hydrogen phosphate, dioctyl phenyl phosphate,didecyl phenyl phosphonate, dihexyl phenyl phosphate,

butyl phenyl phosphonate, and the like. These solvents frequently arediluted with other materials including liquid hydrocarbon, such ashexane, n-heptane, n-octane, or the like. Ethers, such as diethyl ether,dibutyl ether, or ketones, such as diisobutyl ketone, mesityl oxide,diisopropyl ketone, or cyclohexanone or alcohols, such as secondary amylalcohol or isoamyl alcohol or esters, such as isoamyl acetate or butylacetate may be used.

The organic solvent solution of zirconium-hafnium salt thus obtained isobtained quite pure with respect to silica and other metallic componentsand may be used as such as a source for production of zirconium salt.For example, the zirconium salt may be crystallized or otherwiserecovered from the organic solvent.

The organic solution of hafnium and zirconium can be effectivelyextracted with nitric acid in order to separate hafnium from zirconium.For example, it may be introduced into a central stage of a multistageextraction system such as a central area of a vertical extraction columnin which a further portion of the organic solvent is fed into the columnat or near the bottom thereof (one end of the system) and an aqueousacid, such as nitric acid, is fed into the upper portion of the column(the other end of the system), the nitric acid and the solvent beingallowed to countercurrently contact and to extract the solution. Byfeeding the resulting organic solution of zirconium and hafnium into anintermediate point between the feed point of the nitric acid and thefeed point of the organic solvent, it is possible to produce twofractions in the column, one of which is preponderantly of hafnium andthe other preponderantly of zirconium. For example, when an alkylphosphate, such as tributyl phosphate is used as the organic solvent,the zirconium is found to be largely or even substantially entirely inthe tributyl phosphate leaving the top of the column, whereas thehafnium is found to be largely or substantially entirely in the aqueousnitric acid solution leaving the bottom of the column.

According to a further embodiment of the invention herein contemplated,the frit may be preliminarily ex tracted with water in order to remove aportion of the silica. In such a case, the residual solid contains apreponderant amount or substantially all of the alkali metal zirconatetogether with some portion, usually up to about 30 percent of theinitial silica concentration of the frit.

If this wet alkali metal zirconate is contacted with acid, the silicaeither goes into the solution in a colloidal form or is present in sucha finely divided hydrated state that it is extremely slow in filteringand does not separate readily upon settling.

According to this invention, it has been found, however, that if thewater leached product thus obtained is dried adequately and then isadded to the nitric acid under conditions such as to maintain themixture acid as described above, the silica is precipitated in a rapidlyfilterable form which is pulverulent in character as distinguished fromgelatinous.

The degree of drying of the leached frit or leached residue obtained onleaching frit is found to be important. In general, it is foundnecessary to dry this frit at a temperature above 300 C. (rarely over600 C.). If temperatures below about 300 C. are resorted to, the silicawhich is obtained upon treatment of the residue with acid is highlycolloidal and gels the solution when it is contacted with organicsolvent. In contrast, if the residue is heated at a temperature above600 C.800 C., the zirconium content tends to become insoluble and asubstantially reduced amount of zirconium goes into the solution upontreatment with acid.

The following examples are illustrative:

Example I Alkali metal zirconate frit was prepared from zircon ore asfollows:

The zircon sand used contained about 66 percent by weight of zirconiumcalculated as ZrO 1.3 percent by weight of hafnium calculated as HfO and32 percent by weight of SiO The zircon had a particle size below about50 mesh. The zircon was fired with sodium =hydroxide in an externallyfired, rotating tube kiln 26 feet long and having an internal diameterof 3 feet. The kiln was heated to a temperature of about 1050 to 1150 F.

Dry zircon was fed to the kihi, at the entry end thereof, continuouslyat a rate of 6.5 pounds per minute. An aqueous solution containing about50 percent by weight of NaOH was fed into the kiln through 8 sprayswhich were individually supplied by 8 tubes each /4 inch in diameterfrom a common source of sodium hydroxide solution. The first spray waslocated at 5 feet from the feed end of the kiln and the other sprayswere spaced two feet apart in a row downstream of the kiln from thefirst spray nozzle. These tubes were enclosed in a cooling tube 4 inchesin diameter which thus provided a cooling jacket extending along thelength of the kiln. The sprays delivered a downwardly directed flatspray extending longitudinally of the kiln, the angle of each spraybeing about degrees, so that the sprays did not intersect.

The sodium hydroxide was introduced :into the sprays at a total rate ofabout 1.3 gallons per minute, each spray being supplied withapproximately an equal amount of sodium hydroxide solution. Thetemperature of the sodium hydroxide solution was held below the boilingpoint thereof by means of water circulating through the cooling jacketat a rate of about 10 gallons per minute. A dam ring was provided in thekiln to ensure provisions of a bed depth of about 2 to 5 inches of thereacting zircon, so that the caustic was largely consumed before itreached the kiln wall.

The alkali metal zirconate produced was withdrawn from the exit end ofthe kiln at a temperature of about 500 F. Approximately 90 percent byweight of the zirconium in the zircon introduced was converted to sodiumzirconate.

Four hundred and fifty milliliters of nitric acid containing about 70percent by weight of HNO was placed in a flask equipped with awater-cooled reflux condenser and an inlet for adding frit. The flaskwas heated until the nitric acid began to boil and water began tocondense in the condenser.

At this time 170 grams of the alkali metal zirconate frit prepared asabove and cooled to room temperature under an anhydrous atmosphere wasgradually added to the hot nitric acid, the rate of addition beingsufiicient to keep the nitric acid boiling but slow enough so thatsubstantial foaming did not take place. The time of addition was about10 minutes. After the addition was complete, the reaction mixture washeld at boiling temperature and atmospheric pressure for about 30minutes.

The resulting reaction mixture wa a slurry of fine pulverulent silicaand sodium nitrate crystals dispersed in the zirconium nitrate-hafniumnitrate solution and containing about 0.70 pound of dissolved Zr pergallon of slurry.

The resulting slurry was cooled to about F. and was extracted with 1400milliliters of solvent which was a mixture of 50 percent by volume oftributyl phosphate and 50 percent by volume of a liquid hydrocarbonwhich has a boiling point at 760 millimeters of pressure of 170 C. Theresulting organic solvent solution contained 97 percent by weight of thezirconium, 95 percent by weight of the hafnium originally contained inthe slurry and 10 percent by weight of HNO No gelation of the solventdue to the presence of silica took place.

Example II The frit prepared according to Example I was leached withwater. When dried in an oven which was heated to 140 C., constantweight, the material contained 46.4 percent of acid soluble zirconium,8.64 percent of SiO;;, and 14.88 percent of Na O.

In a series of tests, 20 gram portions of the dried cake were calcinedfor one hour at the temperatures indicated in Table I. The dried cakewas then dumped while hot into 130 milliliters of nitric acid heatingunder reflux as in Example I. The initial concentration of the nitricacid was 60-70 percent by weight. The resulting mixture was boiled for15 minutes after the cake was added.

These experiments were run in duplicate. One of each duplicate wasfiltered and the cake washed with a minimum amount of water. The otherduplicate was cooled and then extracted with 400 milliliters of thetributyl phosphate-hydrocarbon solvent described in Example I.

The recovery of zirconium in the first instance was calculated from theparts by weight of zirconium in the filtrate per hundred parts by weightof total zirconium in the sample. Insoluble zirconium was computed bydifference. The silica in the filtrate was measured. The results areshown in Table I.

In the second series of tests, where the solutions were extracted withsolvent, the amount of zirconium extracted into the solvent wascomputed, the amount dissolved in aqueous solution being determined byanalysis. Table II reports the results obtained, the percent solublezirconium in the solvent being computed on the basis of the amount ofzirconium dissolved in the nitric acid and the percent of totalzirconium in the solvent being based upon the amount of zirconiuminitially in the sample.

The results were as follows:

TABLE I Temper- Percent; Pounds of aturc of Soluble Percent Silicon perDrying, Zirconium Insoluble 100 lbs. of C. in Filtrate ZirconiumZirconium in Filtrate TABLE II Temper- Percent Percent; ature of SolubleTotal Drying, Zirconium Zirconium C. in Solvent in Solvent 1 Gelledcompletely.

It will be seen, from the above tests, that products which were dried attemperatures as low as 300 C. gelled completely. In contrast, a goodseparation of the two liquid phases and high recovery was achieved whenthe temperature of calcination was from 500650 C. Even better recoverycould have been achieved with a multistage extraction. On the otherhand, calcination of temperatures above about 600 C. resulted inreduction of solubility in a portion of the zirconium.

Zirconium is preferentially extracted or retained by the tributylphosphate plus diluent phase. The zirconium in an organic solutionprovided according to Example I may be preferentially extracted. Todemonstrate this, solvent feed as produced according to Example I wassubjected to a laboratory separation-extraction as described in ExampleIII.

Example 111 This involved a shakeout embodying the principles of anormal countercurrent separation-extraction process. Three stages,provided by separatory funnels for mixing until phase equilibrium wasreached, were employed. These stages included a feed stage with onestage of scrubbing and one stage of extraction. Thus, by using a dilutenitric acid as scrub, the hafnium was preferentially scrubbed from thesolvent in the scrub stage and solvent containing no metal was used topreferentially extract zirconium in the extraction stage. Arbitraryvolume ratios of 10:2:1 of solvent feed to scrub nitric acid toextraction solvent were used.

Organic solvent feed produced according to Example I containing 0.286pound per gallon of zirconium plus hafnium and 1.3 pound per gallon ofnitric acid was employed. The hafnium and zirconium were present in theweight ratio of 2.2 parts hafnium per parts zirconium. The scrubsolution was 4.75 N nitric acid and the extraction solvent waspre-equilibrated with nitric acid to contain 1.0 pound of nitric acidper gallon of solvent.

After approximate equilibrium was reached, the organic phase in thescrub section contained 0.28 pound per gallon zirconium plus hafniumthat analyzed by weight 1.3 parts hafnium per 100 parts zirconium. Thiswas a 41 percent reduction of the hafnium in one scrub stage. Theaqueous ratfinate after one stage of extraction contained 0.006 poundper gallon zirconium plus hafnium that analyzed by weight 66 partshafnium per 100 parts zirconium. This was a 30 fold enrichment of thehafnium after one stage of extraction.

This procedure was repeated except the scrub acid was 2 N nitric acid.The organic phase in the scrub stage contained 0.04 pound per gallonzirconium plus hafnium that analyzed by weight 87 parts hafnium permillion parts zirconium. This met the hafnium specification for reactorgrade zirconium which is 100 ppm. hafnium maximum. Although thezirconium recovery was not optimum due to overscrubbing by the dilutenitric acid, these results illustrate that, with proper adjustment ofvolume ratios and scrub acid normality, solvent feed (provided byExample I) can be used effectively as feed to the column described incolumn 5 for the separation of hafnium and zirconium. The resultinghafnium rafiinate would be relatively clean and could readily beprocessed to recover the hafnium value.

With aqueous feed there is always a chance for silica and other solidsto accumulate at the interface between the solvent and aqueous phases,even when both streams are free of solids before contacting. Thisphenomenon interferes with extraction operations and eventually leads toupsets. By use of organic feeds such as provided by Example I, theseproblems do not arise or are effectively minimized as attested by thefact that the phase separations were fast and clean throughout all thelaboratory tests.

While one preferred embodiment of the invention has been given above,numerous modifications thereof may be practiced without departing fromthe spirit of the invention. For example, other organic extractants,such as alkyl phosphates and acetates, can be utilized as described inUS. Patent No. 2,757,081. Numerous diluents, such as many petroleumfractions containing aromatics or straight-chain hydrocarbons, can beemployed, depending upon the particular conditions under which thiscolumn is to be operated. In general, it is desired that the diluentgive a maximum difference between the specific gravity of the organicand aqueous phases without unduly complicating the operation of thecolumn from safety, degradation or other consideration. It is notnecessary that the organic phase be lighter than the aqueous phase butonly that they separate readily by gravity. Numerous specific diluentsare hydrocarbons, such as hexane, nheptane, n-octane, the n-alkanes withtwelve, thirteen or fourteen carbon atoms, and methylcyclohexane andcarbon tetrachloride have also given good results. A diluent, which alsohas been successfully used, is a naphtha having a specific gravity of0.75, a boiling point of 167-180 F. and a flash point of 120 F. It issold under the trade name Varsol. A mixture of any of the diluentsenumerated above may also be used.

While the invention has been described by reference to specific detailsof certain embodiments, it is not intended that the invention beconstrued as limited to such details, except insofar as they appear inthe claims.

What is claimed is:

1. A method of separating silica from zirconium in an alkali metalzirconate containing said silica which comprises adding alkali metalzirconate while the silica is in essentially unhydrated undissolvedstate to a pool of aqueous concentrated acid containing enough acid todissolve the zirconium of the zirconate and form an aqueous solutionthereof, maintaining the acid concentration in said pool high enough tomaintain the pH of the pool below about 1 throughout substantially theentire period of alkali metal zirconate addition whereby to hold silicaout of solution and produce a slurry of solid silica in said aqueouszirconium solution and separating the solid silica from the zirconiumsolution.

2. The method of claim 1 wherein the separation of solid silica fromaqueous zirconium solution involves extracting the slurry with a waterimmiscible organic solvent for the zirconium.

3. The process of claim 1 wherein the acid is nitric acid and the alkalimetal is sodium.

4. The method of claim 1 wherein the water content of said pool ismaintained low enough such that the resulting slurry of silica inaqueous zirconium solution contains solid alkali metal salt of the acid.

5. The method of claim 1 wherein the acid concentration of the pool isabove 3 moles of acid per liter of solution.

6. A method of separating silica and zirconium in a frit produced byreaction of zircon and alkali which frit contains alkali metal zirconateand said silica which comprises adding frit containing no more than 5percent absorbed water and while the silica is in an essentiallyunhydrated solid state to a pool of aqueous nitric acid containing atleast 40 percent by weight of HNO to dissolve the Zirconium of thezirconate and form an aqueous zirconium solution, maintaining the acidconcentration throughout the addition above about 30 percent by weightHNO whereby to hold silica out of solution and produce a slurry of solidsilica in said aqueous zirconium solution, and separating undissolvedsilica of the slurry from the aqueous zirconium solution.

7. The method of claim 6 wherein. the water content of the pool ismaintained below about 40 percent by weight and wherein a large portionto the alkali metal salt formed by reaction of the acid with the alkaliprecipitates out of solution.

8. A method of separating silica from zirconium in an alkali metalzirconate which contains alkali metal silicate comprising extracting aportion of the alkali metal silicate from the alkali metal zirconatewith water to provide a water leached residue, heating said residueabove 300 C. and below 600 C. until its silica content is converted toan essentially unhydrated, undissolved state while maintaining zirconiumin the residue soluble in concentrated aqueous acid, contacting the soheated residue with concentrated aqueous acid to dissolve the zirconiumof the zirconate and form an aqueous solution thereof while maintainingthe concentration of said acid solution at a pH of below about 1throughout substantially the entire period of alkali metal zirconateaddition to maintain the silica in the treated residue in asubstantially insoluble state therein to provide an aqueous solution ofzirconium containing pulverulent, readily filterable silica andseparating the resulting solution from said pulverulent silica.

9. The method of claim 8 wherein the concentration of aqueous acid isabove 3 moles of acid per liter of solution.

10. The method of claim 8 wherein the alkali metal is sodium and theacid is nitric.

11. The method of claim 8 wherein the acid is nitric and the so heatedresidue is added to a pool of aqueous concentrated nitric acid while thenitric acid concentration is maintained high enough to maintain the pHof the pool below about 1 throughout substantially the entire period ofresidue addition to dissolve the zirconium of the zirconate and form anaqueous solution thereof containing readily filterable pulverulentsilica.

12. The method of claim 8 wherein the aqueous zirconium solution isextracted with a water immiscible solvent for the zirconium thereby toseparate zirconium from said silica.

References Cited UNITED STATES PATENTS 1/1941 Hixson et al. 2323 4/1956Plucknett 23-312 X H. T. CARTER, Assistant Examiner.

1. A METHOD OF SEPARATING SILICA FROM ZIRCONIUM IN AN ALKALI METALZIRCONATE CONTAINING SAID SILICA WHICH COMPRISES ADDING ALKALI METALZIRCONATE WHILE THE SILICA IS IN ESSENTIALLY UNHYDRATED UNDISSOLVEDSTATE TO A POOL OF AQUEOUS CONCENTRATED ACID CONTAINING ENOUGH ACID TODISSOLVE THE ZIRCONIUM OF THE ZIRCONATE AND FORM AN AQUEOUS SOLUTIONTHEREOF, MAINTAINING THE ACID CONCENTRATION IN SAID POOL HIGH ENOUGH TOMAINTAIN THE PH OF THE POOL BELOW ABOUT 1 THROUGHOUT SUBSTANTIALLY THEENTIRE PERIOD OF ALKALI METAL ZIRCONATE ADDITION WHEREBY TO HOLD SILICAOUT OF SOLUTION AND PRODUCE A SLURRY OF SOLID SILICA IN SAID AQUEOUSZIRCONIUM SOLUTION AND SEPARATING THE SOLID SILICA FROM THE ZIRCONIUMSOLUTION.